1. Field of the Invention
The present invention relates to pulverized coal boilers and, more particularly, to adjustable air foils for balancing pulverized coal flow therein.
2. Description of the Background
In a typical large pulverized coal boiler, coal particulate and primary air flow from the pulverizers to the burners through a network of fuel lines that are referred to as coal pipes.
FIG. 1 illustrates a typical large pulverized coal boiler inclusive of pulverizer(s) 10, furnace 30, and network of coal pipes 20. For proper operation of the boiler, all the coal pipes 20 connected to any one of the pulverizers 10 should carry the same coal flow rates and the same flow rates of primary air.
Unfortunately, differences in coal and primary air flow rates from one coal pipe 20 to the next are a limiting factor in the ability to reduce NOx emissions in pulverized coal boilers. High carbon monoxide emissions and high levels of unburned carbon can result from burner imbalances. High fly ash unburned carbon, in turn, can adversely affect electrostatic precipitator collection efficiency and result in elevated stack particulate emission levels. Imbalances in coal pipe flows can also lead to maintenance problems associated with coal pipe erosion and/or clogging (e.g. excessive localized coal accumulation), damage to burners and windboxes, and accelerated waterwall wastage. Problems such as these reduce the operating flexibility of the boiler and often require that the boiler be operated under conditions which produce higher NOx levels than would otherwise be achieved.
Often, due to the configuration of the boiler system, the flow from a single coal pipe must be split into two or more flows. FIG. 4 shows an example of a four-way splitter arrangement 100 that is sometimes encountered in pulverized coal boiler systems. The arrangement 100 involves coal and primary air flow from a single pipe 102 dividing into four flows at a four-way splitter 104. Industry experience shows that the coal flow rates among the four outlet pipes 106a–d can be severely imbalanced. This is because the distribution of coal flow rates among the pipes 106a–d strongly depends on the pulverized coal flow distribution at the inlet cross-section of the four-way splitter 104, and a significant pulverized coal flow non-uniformity exists due to an upstream elbow 110. The non-uniformity causes the coal particles to stratify into a narrow localized stream (i.e. rope flow) close to the outer wall of the elbow 110. For this reason, a flow splitter must be installed either sufficiently far from an elbow or be designed to accommodate significant coal flow non-uniformity. However, due to the space limitations associated with many applications/installations, a flow splitter has to be installed immediately after an elbow where, as stated above, the coal particulate exists as a narrow, localized rope flow.
The distribution of primary air throughout the coal piping network is controlled by the flow resistances of the various coal pipes 20. Because of differences in pipe lengths and numbers and types of elbows in each fuel line, the different coal pipes from a pulverizer will usually have different flow resistances. It is known that orifices or flow restrictors can be installed within the pipes 20 for use in adjusting the individual primary air flows to make them equal.
For example, U.S. Pat. No. 5,593,131 to O. Briggs and J. Sund shows a Variable Orifice Plate for Coal Pipes for balancing coal pipe flows.
U.S. Pat. No. 5,685,240 to O. Briggs and J. Sund shows a Variable Orifice Plate for Coal Pipes.
U.S. Pat. No. 4,094,492 to R. Beeman and S. Brajkovich shows a Variable Orifice Using an Iris Shutter.
U.S. Pat. No. 4,779,546 to W. Walsh shows a Fuel Line Orifice.
U.S. Pat. No. 5,975,141 to M. Higazy shows an On-Line Variable Orifice.
U.S. Pat. No. 4,459,922 to R. Chadshay shows an Externally Adjustable Pipe Orifice Assembly.
U.S. Pat. No. 6,055,914 to Wark is a pre-riffler mixing device for balancing out the coal and air flows upstream of a riffler box to ensure a more homogenous flow. This is accomplished with concentric mixing rings that interrupt both coal and air flows to create turbulence, thereby mixing the flows. The Wark '914 device restricts the combined coal and air flows, and does not teach or suggest controlling the direction of coal flow distribution into a plurality of outlet pipes without substantially interrupting air flow.
FIG. 5 shows a sub-section of a known existing installation where a Venturi 112 was installed between the exit of the elbow 114 and the inlet of the four-way splitter 116 in an attempt to lower inherent coal flow imbalances. Laboratory testing with this configuration showed a ±35% coal flow imbalance among the four outlet pipes 118.
It can be seen in the above-cited references that orifices with both fixed geometry and adjustable geometry are available commercially.
While the use of fixed or adjustable orifices can be an effective way of balancing primary air flow rates, evidence from field and laboratory measurements indicates the orifices have little effect on coal flow rates. Instead, the coal flow distribution among the pipes is affected most strongly by flow conditions and geometry in the inlet regions of the pipes.
FIG. 2 illustrates a coal pipe 20 according to one piping arrangement commonly encountered in pulverized coal boiler systems. This arrangement involves coal and primary air flow from one pipe 20 dividing into two flows at a Y-shaped junction/splitter. Industry-wide experience shows the coal flow rates among the two outlet pipes 22, 23 can be severely imbalanced. More specifically, conventional orifices 40a–b are installed to prevent primary air flow imbalance and the underlying table shows the results from a series of laboratory tests carried out on the effectiveness of orifices 40a–b. As the data show, selection of the proper orifices 40a–b as required to balance the primary air flow rates did not simultaneously result in a balanced coal flow distribution. In fact, in this case, the orifices 40a–b increased the coal flow imbalance from 9.45% to 18.4%.
A second alternative comprises the insertion of a slotted riffler in a splitter box as shown in FIG. 3 (prior art). The slotted riffler configuration is also commercially used to reduce fuel flow imbalances. The slotted riffler concept consists of a series of flow channels with rectangular cross sections, each of which directs a portion of the coal and primary air flow to one of the outlet pipes. Field measurements show that while these types of rifflers can help to reduce coal flow imbalance arising from a mal-distribution of coal flow at the inlet, they generally do not eliminate the imbalance. Additional fine control of the coal flow distribution is still needed.
A third attempted solution for the coal flow imbalance is the use of adjustable baffles to modify the coal flow distribution among the outlet pipes 22, 23. The following references describe the use of baffles to modify coal flow distribution.
U.S. Pat. No. 4,570,549 to N. Trozzi shows a Splitter for Use with a Coal-Fired Furnace Utilizing a Low Load Burner.
U.S. Pat. No. 4,478,157 to R. Musto shows a Mill Recirculation System.
U.S. Pat. No. 4,412,496 to N. Trozzi shows a Combustion System and Method for a Coal-Fired Furnace Utilizing a Low Load Coal Burner.
Finally, U.S. Pat. No. 2,975,001 issued on Mar. 14, 1961 to Davis discloses an apparatus for dividing a main stream of pulverized coal between two branch streams. (Col. 1, lines 50–52). The apparatus may be used alone or in conjunction with a conventional slotted riffle. (Col. 1, lines 70–73). The apparatus is comprised of a combination fixed and tiltable nozzle. (Col. 1, lines 50–58). The fixed nozzle is attached to the main duct leaving the pulverizer and concentrates the coal and air flow. (Claims 1–5). The concentrated coal and air flow is then directed into the tiltable nozzle with the highest concentration of coal necessarily being at the nozzle centerline. The tiltable nozzle is then “tilted” in order to direct the concentrated coal and air flow into one or the other branch stream. (Claims 1–5).
Guide vanes may be mounted inside the tiltable nozzle; however, this patent does not disclose adjustable guide vanes. (Col. 1, lines 58–60).
All of the foregoing references teach a form of direct diversion of both the coal and air flow. It is impossible using direct diversion to increase or decrease the flow of coal into a particular outlet pipe without effecting primary air flow, or vice versa.
According to Schlichting's Boundary Layer Theory, McGraw Hill, 7th ed, 1979, a wake is formed behind a solid body which has been placed in a stream of fluid. The axial velocities in a wake are smaller than those in the main stream. As the downstream distance from the body is increased, the differences between the velocity in the wake and that outside the wake become smaller. The present inventors specifically avoid direct jet diversion of the entire flow stream as described in Davis '001, and instead use airfoils to form wakes to indirectly divert the coal flow without affecting primary air flow. The difference is significant because the gas and particle flow in the wake region, a short distance downstream, has the lowest particle concentrations and velocities and air velocities at the centerline behind the object. Used with a riffler as described above, this makes it possible to increase or decrease the flow in one of the outlet pipes by moving the wake-inducing foils in a direction perpendicular to the flow. The unique approach makes it possible to increase or decrease the flow of coal into a particular outlet pipe without effecting primary air flow. In contrast, it is very difficult with an adjustable baffle approach to simultaneously balance coal and primary air flow rates.
It would, therefore, be advantageous to provide splitter designs that eliminate coal flow imbalances at crucial points in a pulverized coal boiler system using an on-line adjustment capability that does not disturb any pre-existing primary air flow balance among the multiple coal pipes. This would permit the operation of the pulverized coal boiler system to be optimized and result in reduced pollutant emissions and improved combustion efficiency.